1. Field of the Invention
This invention relates to a failure diagnostic apparatus in an engine air intake system for judging a failure of an air flow sensor adopted for detecting an intake air amount in the engine air intake system, which controls the intake air amount by opening and closing a throttle valve arranged in an intake pipe of an engine for a vehicle.
2. Description of the Prior Art
It is known that engine output control in a vehicle is performed by opening and closing a throttle valve disposed in an intake pipe for changing an intake air amount or a fuel amount for a cylinder.
There have been provided two types of throttle valve system in the air intake pipe for practical application. One is a mechanical throttle system where the throttle valve is opened and closed in association with a depressing operation of an accelerator pedal. The other is an electrically powered throttle system where a target throttle opening is set on the basis of a depressed amount of the accelerator pedal and the opening and closing operation of the throttle valve is automatically controlled according to the target throttle opening. The electrically powered throttle system is provided with a throttle opening sensor for detecting a throttle opening of the throttle valve. In the electrically powered throttle system, a deviation between a throttle opening actually detected by the throttle opening sensor and the target throttle opening is determined. An electrically powered actuator is controlled so that the deviation becomes "0", thereby adjusting the throttle opening to the target throttle opening.
In this case, the throttle opening sensor lacks high reliability, as being of a contact type. If the throttle opening sensor is in trouble, a feedback control is incapable because the actual throttle opening for the target throttle opening becomes unavailable.
Therefore, it is proposed that the intake air amount of the intake pipe, which is changed by controlling the throttle opening, is detected by an air flow sensor of a non-contact type having a high reliability and the throttle valve is controlled so that the intake air amount detected by the air flow sensor is coincided with the target intake air amount.
The air flow sensor should be monitorred whether it works normally, as being employed as an important means for detecting an engine condition because of its high reliability.
There has been proposed a judging means for judging a failure of the air flow sensor, which is useful in the event that no output signal is available from the air flow sensor although the engine is in operating state.
However, since the failure judging means above described judges only whether an output signal is output from the air flow sensor, the failure judgement cannot be made in the intermediate state on the intake air amount which changes during travelling state.